Because of the capability of recording and reproducing three-dimensional stereoscopic image, a hologram has been used for cover paper for book, magazine and the like; display for POP and the like; gift and the like by making use of its excellent design and decorative effect. Further, because of its capability of recording fine data in a submicron unit, a hologram has been applied to mark for preventing forgery of securities, credit cards, prepaid cards, and the like.
In particular, a volume phase hologram forms spatial interference bands having different refractive indexes in a hologram recording material to allow the modulation of light transmitted by the hologram thus formed and thus has been expected to be applied to scanner for POS and holographic optical element (HOE) such as head up display (HUD) besides display.
In order to meet requirements for such a volume phase hologram, a volume phase hologram recording utilizing a photopolymer has been so far proposed.
Specifically, as a process for the production of a hologram employing a photopolymer, there has been proposed a process which comprises exposing a hologram recording material composed of a photopolymer to an interference pattern of radiation, and then subjecting the hologram recording material to development with a developer.
For example, there is disclosed a process for the production of a hologram employing a photopolymer, which comprises a first step of exposing a photographic material comprising a polyfunctional monomer having two or more ethylenically unsaturated bonds and a photopolymerization initiator incorporated in combination in a polymer as a carrier to an interference band of radiation, a second step of treating the photographic material with a first solvent so that it swells with the solvent, and a third step of treating the photographic material with a second solvent having a poor swelling effect so that it shrinks (see Patent Document 1).
In accordance with this conventional technique, a hologram excellent in diffraction efficiency, resolution, environmental resistance, etc. can be produced. However, this method is disadvantageous in that the hologram has poor sensitivity properties or poor properties of sensitive wavelength and it is subject to production complicatedness such as employment of wet step and problems such as deterioration of transparency due to development unevenness or whitening caused by void or cracking generated during dipping in solvent.
On the other hand, there are disclosed a hologram recording material employing a photopolymer capable of producing a hologram by interference exposure alone as only treatment step, which does not require complicate or troublesome wet step during the production of the hologram, and a process for the production thereof. For example, a hologram recording photosensitive layer comprising an aliphatic polymer binder, an aliphatic acryl monomer and a photopolymerization initiator is proposed (Patent Document 2).
However, this conventional technique is disadvantageous in that the high molecular polymer and aliphatic acryl monomer used have close refractive indexes and the modulation of refractive index attained by hologram exposure thus ranges from 0.001 to 0.003, making it impossible to obtain a high diffraction efficiency.
Further, it has heretofore been attempted to raise the refractive index of a polymethyl methacrylate (PMMA), which is known as a representative optical polymer, only by irradiation of light, without introducing any low molecular compounds. Although this technique can provide a refractive index difference of 0.051, which is great enough for optical devices, in the case of being irradiated with light of 325 nm, it is disadvantageous in that in order to provide PMMA with reactivity, methyl methacrylate which is a monomer has been previously oxidized before being polymerized, requiring a long period of time to prepare PMMA and complicated steps.
It has been reported that, when methyl methacrylate which is a monomer has not been previously oxidized before being polymerized, the refractive index of PMMA shows no rise even when irradiated with the aforementioned light (see Non-patent Document 1). Further, it is reported that, when the wavelength of the light with which PMMA is irradiated is lowered, e.g., to 0.2537 μm, there is a tendency that the main chain of PMMA is severed to reduce the density thereof (see Non-patent Document 2), suggesting that the refractive index of PMMA cannot be raised from the standpoint of Lorenz-Lorenz equation.
Further, with regard to inorganic materials, a method which comprises irradiating glass doped with germanium with light so that the refractive index thereof varies to prepare a light diffraction lattice has been known. Moreover, with regard to polymer materials, a technique which comprises irradiating a material having a photochemically reactive low molecular material dispersed in a polymer with laser beam so that a photochromic reaction (photobleaching) is induced accompanied by the change of refractive index to prepare a light diffraction lattice has been disclosed (see Patent Document 3). Moreover, a technique which comprises using the aforementioned photobleaching to produce a so-called refractive index-distributed material (GRIN material) having a continuous change of refractive index therein has been disclosed (see Patent Document 4).
These conventional techniques involve the use of a material doped with a low molecular material or a polymer molecule having a low molecular material incorporated therein, and in some cases, light absorption by the low molecular material increases, occasionally making it impossible to obtain a sufficient device transparency.
Patent Document 1: JP-B-62-22152
Patent Document 2: U.S. Pat. No. 3,658,526
Patent Document 3: JP-A-7-92313
Patent Document 4: JP-A-9-178901
Non-patent Document 1: M. J. Bowden, E. A. Chandross, I. P. Kaminow, “Applied Optics”, vol. 13, p. 113 (1974)
Non-patent Document 2: W. J. Tomlinson, I. P. Kaminow, E. A. Chandross, R. L. Fork, W. T. Silvast, “Applied Physics Letters”, vol. 16, p. 486 (1970)